1. Field of the Invention
The present invention relates to a power transmission apparatus which can continuously change a ratio of a rotational speed and a vehicle which performs a variable speed travel with using the power transmission apparatus.
2. Description of the Prior Art
In a vehicle which employs an internal combustion engine as a main power source, since a wide variable speed travel is requested between a low speed travel at a starting time and a high speed travel, a speed change gear is employed for the power transmission apparatus. As a conventional speed change gear for a vehicle, there have been known a discontinuously variable transmission which replaces an engagement of a gear so as to change a shift ratio step by step., and a continuously variable transmission which can continuously change a shift ratio. Then, in view of a comfortable drive, the continuously variable transmission having no shift shock is desirable.
As the conventional continuously variable transmission, for example, there has been known a continuously variable transmission mechanism constituted by a plurality of conical rollers and connecting rollers arranged between respective conical rollers, as described in Japanese Patent Unexamined Publication No. 7-127702. Further, as another continuously variable transmission, for example, there has been known a gear type continuously variable transmission constructed by combining a conical helical gear and a relay gear, as described in Japanese Patent Unexamined Publication No. 1-303358. Further, a belt type continuously variable transmission has been also known.
However, for example, in the continuously variable transmission described in Japanese Patent Unexamined Publication No. 7-127702, a friction gear mechanism is employed as a base for transmitting a power, so that there is a problem that a power transmission loss is greater than the discontinuously variable transmission. Because in order to transmit a great torque, it is necessary to apply a great force in a direction perpendicular to a direction of transmitting the power for increasing a friction force, whereby a loss of a hydraulic mechanism for generating the force is increased.
Further, in the conventional belt type continuously variable transmission, since a friction loss is also generated due to a slip at a time when a belt is engaged and disengaged, there is a problem that a transmission loss is further increased.
On the contrary, in the gear type continuously variable transmission described in Japanese Patent Unexamined Publication No. 1-303358, since a helical gear is employed, a slip is always generated in a direction perpendicular to a direction of transmitting the torque during the operation, so that there is a problem that a friction loss is great.
As mentioned above, in the conventional continuously variable transmission, there has been a problem that the power transmission loss is great. Further, in the vehicle employing the conventional continuously variable transmission, since the power transmission loss is great in the continuously variable transmission, there has been a problem that a specific fuel consumption is deteriorated.
A first object of the present invention is to provide a power transmission apparatus having a reduced power transmission loss.
A second object of the present invention is to provide a vehicle having an improved specific fuel consumption.
In order to achieve the first object mentioned above, in accordance with the present invention, there is provided a power transmission apparatus comprising:
a first gear having a first rotational axis and a cross sectional shape which continuously changes along a direction of the rotational axis; and
a second gear meshing with the first gear and having a thickness smaller than that of the first gear, wherein a radius of a pitch circle of one of the first gear and the second gear continuously change when the second gear slides with respect to the first gear while changing a position of engagement.
In accordance with the structure mentioned above, since the second gear slides with respect to the first gear, whereby the radius of the pitch circle continuously changes, it is possible to continuously change a shift ratio and reduce a power transmission loss.
In accordance with a second aspect of the present invention, there is provided a power transmission apparatus as recited in the first aspect, wherein in preferable, the first gear has a cross sectional shape in which a radius of the pitch circle continuously changes along the direction of the first rotational axis.
In accordance with a third aspect of the present invention, there is provided a power transmission apparatus as recited in the first aspect, wherein in preferable, the first gear has a cross sectional shape in which a pressure angle continuously changes along the direction of the first rotational axis.
In accordance with a fourth aspect of the present invention, there is provided a power transmission apparatus as recited in the first aspect, wherein in preferable, a cross sectional shape of the first gear is a cycloid tooth, the second gear has a cycloid tooth, a radius of an outer rolling circle forming the second gear is equal to a radius of an inner rolling circle of the first gear, and a radius of an inner rolling circle forming the second gear is equal to a radius of an outer rolling circle of the first gear.
In accordance with a fifth aspect of the present invention, there is provided a power transmission apparatus as recited in the first aspect, wherein in preferable, a cross sectional shape of the first gear is an involute tooth, the second gear has an involute tooth, and the first gear is structured such that a pressure angle continuously changes along the predetermined path.
In order to achieve the second object mentioned above, in accordance with the present invention, there is provided a vehicle having a power source, a power transmission for changing and transmitting a rotational power generated from the power source, and tire wheels driven by the rotational output shifted by the power transmission apparatus, the power transmission apparatus comprising:
a first gear having a first rotational axis and a cross sectional shape which continuously changes along a direction of the rotational axis; and
a second gear meshing with the first gear and having a thickness smaller than that of the first gear,
wherein a radius of a pitch circle of one of the first gear and the second gear continuously change when the second gear slides with respect to the first gear while changing a position of engagement.
In accordance with the structure mentioned above, it is possible to reduce a power transmission loss of the power transmission apparatus and improve a specific fuel consumption of the vehicle.